1. Field of the Invention
The present invention relates to the determination of the vapor pressure of a single compound liquid having a single vapor pressure or a mixture of liquid compounds having different vapor pressures. An example of a single compound liquid is propane; an example of a liquid mixture composed of different compounds with different vapor pressures is crude oil. The present invention determines the highest pressure of the liquid in a given temperature at which the lightest component starts to flash, such pressure being the vapor pressure of the liquid at that temperature.
2. Prior Art
The determination of the correct vapor pressure in pure liquids or liquid mixtures is very important for the processing of such liquids. One reason for measuring vapor pressure of liquids is to assure that the available net positive suction head of a pump is not less than the required net positive suction head to prevent cavitation and pump damage. Another reason for measuring vapor pressure is to determine how much vapor would escape through vents in storage tanks. Still another very important use of vapor pressure analyzers is to determine the flash point of flammable liquids to determine how safely they can be handled.
Present devices exist to measure vapor pressure of liquids such as Boyd's and Hills' apparatus, U.S. Pat. No. 3,056,282 which is composed of a venturi nozzle where the liquid is passed through. The pressure decreases as the liquid passes through the throat of the nozzle causing it to flash at some pressure existing at the throat which is claimed to be the vapor pressure of the liquid. For single compound liquids this may be accurate but only when not all of the liquid has flashed. Since there is no control on the amount of liquid that is flashed or on how much to decrease the pressure of the sample to reach the vapor pressure, accuracy cannot be ascertained. For multi compound mixtures it is even worse because the pressure should only be dropped enough for the lightest fraction to begin to flash. In a venturi the pressure will continue to decrease after initial flashing because there is no controller to stop further expansion once initial flashing is obtained. The liquid mixture will continue to flash even after all the lightest fraction has flashed because flashing will only stop when equilibrium pressure is reached.
Other devices that attempt to measure multi component liquids having different boiling points do so either using the same principle as for the single component liquid or relies on a sampling technique that analyzes the sample in batches such as Reed's apparatus in U.S. Pat. No. 5,172,586. In a batch sampling technique, the sample of the multi component liquid is accumulated in a cylinder. It is then expanded in steps in the chamber and the pressure is monitored. Since the fluid is analyzed in batches instead of continuously, the vapor pressure of the flowing liquid is not measured while the sample is still being analyzed. Also, this device waits for the sample to stabilize which can increase the length of the sampling interval and therefore the device cannot monitor changes in vapor pressure occurring in the system during the analysis interval. Sampling time is 30 to 300 seconds. Also, the space occupied by the liquid is expanded by 20% initially which drops the pressure in the chamber below the vapor pressure of the sample. This means that it cannot be ascertained what the vapor pressure of the sample is because at equilibrium conditions with 120% of initial volume, all the lightest fraction may already have flashed and the equilibrium pressure is then at a lower pressure than it would have been if the sample were only expanded to a point as to initially flash the lightest fraction. Moreover, linear interpolation will not likely yield the true vapor pressure. Another disadvantage of these prior arts is that accuracy can be affected by dirt or solid accumulation inside the sampling chamber. After a certain interval the parts have to be taken off line, dismantled and cleaned.